Boulder Creek fish kill caused by low water, high temps

A dead fish lies on the bank of Boulder Creek just west of 28th Street last month. (Paul Aiken)

A low level of dissolved oxygen, caused by a combination of high water temperatures and a weak stream flow, led to the deaths of more than 250 fish in Boulder Creek last month, city officials reported Tuesday.

The portion of the creek -- between the Millennium Harvest House hotel and 28th Street -- where 263 brown trout and white suckers died Aug. 20 was extremely shallow that day, city spokeswoman Jody Jacobson said.

She said depleting snowmelt at high elevations and upstream water rights owners pulling water from the creek likely caused the low stream flow. Water in that section of Boulder Creek was measured to be flowing at just 1 cubic foot per second, and temperatures in the creek topped out at 66 degrees.

Typical flows near the hotel in late summer usually don't dip below 5 cfs, said Carol Ellinghouse, water resources coordinator for the city of Boulder.

Initial reports indicated that the die-off might have been the result of a fine sediment discharge into the creek from a stormwater outlet at Folsom Street. But Jacobson said the Colorado Division of Wildlife didn't find any dead fish near that discharge pipe, which is upstream of the hotel.

At most, she said, sediment would have been a contributing factor to the low dissolved oxygen level in that portion of the creek, but not the cause of the fish kill.

Ellinghouse said a natural -- but dramatic -- decrease of about 20 cfs was measured in Boulder Creek's flow at Boulder Falls on Aug. 20.

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She called the drop an unusual "natural phenomenon" but couldn't say specifically what might have caused it. Officials traveled upstream of Barker Dam in Nederland to see if there had been any illegal diversions of water but found none, she said.

Ellinghouse said the state water commissioner for Boulder Creek, who controls how much water is diverted by rights holders along the waterway, was unable to react quickly enough to the sudden drop in stream flow Aug. 20 to prevent the fish kill.

"It was a very dramatic drop," she said. "He assumed there was a larger influx of water into the river."

The commissioner adjusts the amount of water taken from the creek every 24 hours, and it takes time for flows to be restored after ditch companies and other water users are ordered to re-position their head gates to divert less water, Ellinghouse said.

"It's a monumental job," she said. "It's a very complicated management of an entire water system."

She said the fish kill happened where it did because Boulder Creek flattens and widens out at 28th Street and the soils become more porous, absorbing more water into the aquifer.

Ellinghouse said the city has seen fish die off in that part of the creek on previous occasions.

"It's not uncommon to have fish die-offs in August because it is a snowmelt-driven stream," she said. "But this was remarkable because of its magnitude."

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